In the age of introduction of digital film processing and digital cinema, multiple digital processing steps appear in the classically analogue cinematographic workflow. While digital special effect generation and post-production have been common for some time, digital cameras and new digital display devices and digital media become more and more widespread. The digital equipment completely changes the analogue habits during capture and post-processing. While digital technologies bring new features, they also represent a danger for artistic experience and heritage. It is thus necessary to transfer known artistic effects from conventional cinematographic post-production into digital cinema post-production. Since the introduction of digital cinema is long term, technologies for co-existing analogue and digital processes also have to be studied. This may for example concern a film captured by an analogue film camera and then displayed by a digital projector, or integration of digital special effects in an analogue film production chain.
One artistic workflow is colour correction, which is applied during post-production to raw film material to compensate for illumination colour artifacts. Colour correction is also applied before film distribution to fine-tune the colour tones in order to realise artistic intent for certain scenes. Colour correction is further applied to photographs, paintings or graphics before printing. Colour correction may be applied for a sequence of video frames, for a single video frame, for still images or even for parts of an image, such as an object; for example, in photographs, colour correction is commonly used to remove “red” eyes.
Colour correction is usually performed in cooperation between the artistic director and highly skilled operators. The artistic director describes the intent of colour correction while the operator transforms the intent into a colour transform applied to the visual content. During this process, the artistic director and the operator have to consider the impact of the applied colour correction. Prior art colour correction is usually only applied to a current content subset—e.g. a single image or a single visual object in a single image of the visual content, such as a film, a set of associated images or other type of visual support. The colour correction is chosen such that the current content subset gives the aimed appearance of the current content subset. Thereafter, the chosen colour correction is often applied to a number of images of the same or similar object in a number of images.
Since colour correction does not take into account the totality of visual content at a time, a number of problems may arise.
A first problem may arise when colour correction is applied to a single image or to a number of images showing related scenes, such as a shot in a film. The corrected image or images may then appear in an unwanted manner when compared to other images, such as the following shot of the film. In other words, colour correction carried out independently for different shots may result in unconsidered and unwanted colour effects at shot transitions.
A second problem may arise when colour correction is chosen and applied to a single object in a single image or in a number of images showing related scenes. The same colour correction applied to all occurrences of the object in the visual content may lead to unwanted colour appearances in other images of the visual content than those used for choosing the colour correction. This may for example happen when the composition of colours in the other images is different from the composition of colours in the images used for the choice the colour correction.
It can therefore be appreciated that there is a need for an improved method and system for colour correction of image sequences.